docs: def resolve_meta_key(hub, key, meta):
      """ Resolve a value when it's a string and starts with '>' """
      if key not in meta:
          return None
      value = meta[key]
      if isinstance(value, str) and value[0] == '>':
          topic = value[1:]
          if topic not in hub:
              raise KeyError('topic %s not found in hub' % topic)
          return hub[topic].get()
      return value